Oil and gas wells normally employ steel casing as a conduit for produced or injected substances. In recent years, many operators have begun to re-enter and sidetrack existing wells to take advantage of newer technologies such as horizontal and underbalanced drilling techniques. The existing practice requires that a gyroscopic directional survey of the cased well be conducted to establish an accurate profile of the well and a starting point for the sidetrack. Steel casing disrupts the earth's natural magnetic field and precludes the use of directional measurement devices which depend on the earth's magnetic field as a reference. State of the art gyro systems employ costly earth rate gyroscopes and surface readout features which dictate the requirement for electric conductor wireline equipment as well.
Once the well has been surveyed, a bridge plug and a casing whipstock are located at the sidetrack point and oriented in the desired direction of deviation. If the well is vertical or near vertical, the whipstock is oriented using the gyro surveying equipment. A series of milling tools are used to machine a slot in the casing and thereby create an exit point or window. A drill bit driven by a downhole mud motor equipped with a bent housing member is employed to deviate the new wellbore in the desired direction.
In vertical or near vertical wells, a gyroscopic orienting instrument is once again required to orient the motor toolface in the same direction the whipstock was aligned. Since gyroscopic instruments are not built to withstand the shock forces encountered while drilling, the gyro is pulled up into the drill pipe before drilling commences. As drilling progresses, operations must be halted periodically to check the motor's toolface orientation with the gyro. Moreover, these checks are done in a static condition which does not give an accurate indication of reactive torque at the bit and therefore requires the operator to extrapolate the actual toolface orientation while drilling. Drilling must continue in this manner until enough horizontal displacement has been achieved in the new wellbore to escape the magnetic effects of the steel casing on a magnetically referenced orienting device such as a wireline steering or a measurement while drilling (MWD) tool. Alternatively, drilling must continue until enough angle has been built to allow the use of a steering tool or MWD-based gravity referenced orienting device. Only at this point can the gyro and wireline equipment be released and the more cost effective and operationally superior MWD tool be employed.
This conventional method of steering a sidetracked well in the vicinity of steel casing has two disadvantages. First, the requirements for gyroscopic survey equipment and electric conductor wireline equipment add significant cost to the operation. During the time that milling operations are in progress, this equipment is normally kept on standby. Once drilling begins, the actual operating time of the gyro survey equipment is minimal even though the time to release of its services may be substantial. The gyro service incorporates highly sensitive equipment which commands high service charges and, along with the wireline service, requires two or three operations personnel to operate the equipment.
The second disadvantage of the prior art methods relates to their accuracy. The orientation method is inferior as it normally incorporates static instead of dynamic survey data. In operation, the gyro is seated in the muleshoe with the rig's mud pumps turned off. The motor toolface is oriented in this condition and the gyro is pulled up into the drill string before the pumps are started and drilling commences. During drilling, the drill bit's interface with the formation generates reactive torque which causes the orientation of the motor toolface to rotate counterclockwise from its initial setting. Although numerous orientation checks may be made to determine the effects of reactive torque, the gyro equipment cannot be used to obtain orientation data while drilling is in progress. Data obtained must be extrapolated and assumed values used to correct for reactive torque. Since the severity of reactive torque is a function of drill bit torque, drillers normally use low bit weights while orienting with gyro equipment in order to minimize effects on the toolface orientation. This results in slow penetration rates and even higher costs associated with the sidetrack procedure.